Preheat oven to 180°C.
Heat the oil in a small frypan and sauté the onion and chillies for 2 to 3 minutes. Add the tomatoes, vinegar, and water; simmer for 15 minutes, until the onions become a soft, jamlike consistency.
Combine the chicken, oregano, lemon zest, lemon juice, salt, and pepper in a large bowl. Using your hands, mix well. With damp hands, mould mixture into 8 small patties.
Heat a well-oiled grill pan to hot. Sear patties on each side for 2 minutes.
Transfer to a baking tray and bake in oven for 7 minutes.
Serve patties on open bread rolls and top with salad leaves, chilli-onion jam, and mashed avocado.